This invention relates generally to the field of starch production. More specifically, this invention relates to the manufacture of a boiling-stable granular resistant starch.
Starch is composed of two polysaccharides, both of which are glucans, or polymers of glucose. One is amylose, a linear fraction with glucose units joined by a 1-4 glycosidic bond. The other is amylopectin, a branched component where each branch is relatively short and contains about 20-30 glucose units. Starches from different sources are characterized by different relative proportions of the amylose/amylopectin. Depending on the source of the starch, it will contain about 20-28% amylose.
Amylose consists of 250-300 D-glucose units linked by xcex1-1,4-glucosidic bonds. These bonds tend to twist the chain into a helix. In amylopectin, the majority of the units are similarly connected by xcex1-1,4-glucosidic bonds, with occasional xcex1-1,6-glucosidic bonds.
Before starches can be absorbed by the intestinal epithelium, they must be hydrolyzed to their constituent monosaccharide subunits. This cleavage occurs sequentially in different parts of the gastrointestinal tract. Enzymes are first secreted in the saliva (an xcex1-amylase [ptyalin]) and subsequently as another xcex1-amylase in the pancreatic juice. Digestion is continued and completed in the small intestine. Both salivary and pancreatic amylases are 1,4-glucosidases and serve to hydrolyze only the 1,4-glucosidic bonds found in starch and glycogen. Enzymes which attack 1,6-glucosidase linkages are associated with endothelial cells of the small intestine.
Until recently, starch was believed to be fully digested in the small intestine. It is now known that the physical state of ingested starch can result in incomplete digestion in the small intestine.
Undigested starch reaching the large intestine may be fermented to a variable extent. Fermentation products include volatile fatty acids (butyrate, propionate, acetate), which may be absorbed by the colonic epithelium and either metabolized or transferred to the liver through the portal vein. Non-fermented starch appears in the feces.
Resistant starch (RS) is resistant to digestion by xcex1-amylase, and has been defined as xe2x80x9cthe sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals.xe2x80x9d Even though RS escapes digestion in the small intestine, it may be fermented in the large intestine by colonic microflora. In the last decade, there has been an increased interest in the nutritional implications of RS, not only because of its decreased caloric content but also because RS may have a similar physiological effect as dietary fiber. Moreover, the fact that processing treatments may alter RS content in foods has gained the attention of food technologists.
RS has been classified in four different categories: (1) type I, resulting from physical inaccessibility in intact tissues or other large particulate materials; (2) type II, resulting from the physical structure of the uncooked, native starch granules, especially potato, banana, and high-amylose maize starch; (3) type III, resulting from the physical structure of associated starch molecules after the starch granules are cooked; and (4) type IV, resulting from chemical modification that interferes with the enzyme digestion.
Type I RS is easily understood and can be readily lost in processing treatments. Type IV RS results from covalent modification of the starch substrate such that the xcex1-amylase can not approach the susceptible glycosidic linkages. Based on the general inclination of food manufacturers to avoid xe2x80x9cmodified starchxe2x80x9d on the label of the label of a formulated food, intentional generation of RS by chemical modification is not preferred.
Although the four types of RS would suggest four approaches to manufacture RS, only the approach for manufacturing type III RS has been extensively studied. (Pomeranz 1992; Gidley et al. 1995; Eerlingen and Delcour 1995). The approach for manufacturing type IV RS is somewhat limited by the type and extent of derivatization that may be legally used in foods. Little information about strategies to improve the manufacture of types I and II RS exists in the literature.
Some varieties of starches are good sources of type II RS (as determined by the procedure of Euglyst et at. 1992), for example, banana (69-89% RS), potato (80-87% RS), and high-amylose maize starches (HAMS)(55-85% RS). However, the enzyme resistance of these starches is highly reduced (HAMS) or completely lost (banana and potato) after moderate heat processing conditions, such as boiling in excess of water. The lack of thermal stability of type II RS represents a limitation for use of food ingredients with type II RS.
On the other hand, type III RS is considered to be thermally stable. The thermal stability of type III RS has made it a suitable additive for uses in many foods intended to contain RS. Several manufacturing processes have been developed to increase the yield of type III RS. In these processes, the preferred material has been high-amylose maize starch (HAMS). The most common commercially available types of HAMS are not gelatinized by boiling in excess water at atmospheric pressure. The HAMS""s are autoclaved (at 121xc2x0 C. or higher) for production of type III RS. Additional treatments, including limited acid or enzyme hydrolysis before or after autoclaving have been used to enhance the yield of RS. U.S. Pat. No. 5,281,276 (Chiu, et al). After gelatinization by autoclaving, however, the material is no longer in granule form.
Unlike the type II RS of potato or banana, the type II RS of some types of HAMS is not completely lost on boiling at 100xc2x0 C. Many investigators have employed the AOAC method for Total Dietary Fiber (TDF) as a means of preparing RS for further analysis. [Sievert, D. et al. (1989). Enzyme-resistant starch. I. Characterization and evaluation by enzymatic, thermoanalytical, and microscopic methods. Cereal Chem. 66(4): 342-347]. This analytical procedure involves simultaneous boiling and thermostable xcex1-amylase digestion. Although this procedure was designed for TDF, a fraction of the RS will contribute to the putative TDF. For native HAMS as well as from various sources of type III RS, when the starch sample is subjected to the TDF analytical procedure, no other source of TDF is present at the start, and purified RS may be isolated. However, by this approach no RS can be isolated from potato or banana starch, because for these starches the granule structure and its constituent resistant elements are lost due to the boiling treatment (even for HAMS, much of the original type II RS is lost during this analysis). Thus, HAMS is unique in that it is a source of heat-resistant type II RS, which by definition is in granular form. A limitation of native HAMS is that the proportion of heat-resistant type II RS is relatively low.
The thermal stability of type II RS has been enhanced by hydrothermal treatments. Annealing and heat-moisture treatments have been considered two types of hydrothermal treatments that can modify the physicochemical properties of starch without destroying the granule structure. Both treatments involve incubation at certain moisture levels and temperatures above the glass transition temperature and below the gelatinization or melting temperature. Heat treatments at high-moisture levels have been termed xe2x80x9cannealingxe2x80x9d (ANN) while treatments performed at low-moisture levels have been termed xe2x80x9cheat-moisture treatmentsxe2x80x9d (HMT).
The structures responsible for type III RS formation are thought to be based on junction zones built up from associated double helices primarily from regions of amylose, but also possibly from the longer chains of the unique amylopectin of HAMS. The length of acid-resistant regions has been estimated as from 40-80 AGU. (Jane J. L. et al. (1984), Structure studies of amylose-v complexes and retrograded amylose by action of a-amylases and a new method for preparing amylodextrins. Carbohydrate Research 132:105-118.) The structures responsible for type II RS are not understood, but it is reasonable to suppose that the RS can also be accounted for at least partially by double helices that are associated with other double helices.
Acid hydrolysis after thermal processing has been used to increase the proportion of RS in the product by selectively removing non-RS material. (Chiu et al., U.S. Pat. No. 5,281,276.) Others have recently shown ANN followed by partial acid hydrolysis can result in an increased proportion (about 30%) of granular RS from HAMS. Haralampu et al., U.S. Pat. No. 5,849,090.
The present inventors have now surprisingly discovered that even higher yields of RS are possible when the sequence of ANN followed by acid hydrolysis is reversed, with acid hydrolysis followed by the hydrothermal treatment.
It is therefore an object of the present invention to provide a method and means of increasing the yield of thermally stable RS.
It is yet another object of this invention to provide a method and means of increasing the yield of thermally stable RS using a hydrothermal treatment.
It is yet another object of this invention to provide a method and means of increasing the yield of thermally stable RS in granule form.
It is yet another object of this invention to provide a method and means of increasing the yield of thermally stable RS which uses the acid-resistant portions of the starch granule as a template for enhancing enzyme resistance.
Other objects of the invention will become apparent from the description of the invention which follows.
The present invention describes the use of partial acid hydrolysis of granular starch followed by hydrothermal treatment to produce granular resistant starch (RS) that is stable to further heat treatment at atmospheric pressure. Partial acid hydrolysis causes partial depolymerization of the starch, making it more susceptible to hydrothermal treatment. The acid hydrolyzed starches are then treated by annealing or heat-moisture treatment that modify the physicochemical properties of the starch without destroying its granular structure. The resulting granular RS has an unexpectedly high yield of RS (greater than 60%) in comparison to RS produced by hydrothermal treatment followed by limited acid hydrolysis (about 30%).